Galls develop when leaves are sucked by the juveniles, which meta morphose into adults that erupt out of the gall (remember the movie ‘Alien’?). Click here for more psyllid information.
There is a significant lerp infestation on Foxes Lair wandoos at the moment.
When I opened a few to have a look, I found maggots instead of psyllids. The maggots are larvae of the gall-midge (in actual fact a Cecidomyidae fly) that produces saliva, which forces acacias to produce gall cells. These have replaced flowers making the plant look like it has fruit.